1. Field of the Invention
The present invention relates to a method for injection stretch blow molding of polyethylene into a hollow molded product, such as a thin-wall container.
2. Background Art
In an injection stretch blow molding method comprising injecting molten resin into an injection mold to mold a preform in a blow mold, and stretch blow molding the perform into a hollow molded product such as container, it is possible to mold almost all thermoplastic resins into hollow thin-wall products. At the present, however, the method's used is limited to polyethylene terephthalate, polypropylene, polycarbonate, vinyl chloride, etc., and not applied to polyethylene which is high demand for hollow molded products.
Known methods for molding continuously from injection molding of preform to stretch blow molding of hollow molded products include a method comprising releasing an injection molded preform from an injection mold while the preform is still hot before being completely cooled and solidified, conditioning the temperature of the hot preform, and stretch blow molding the conditioned preform into a hollow molded product in a blow mold, and also a method of immediately stretch blow molding by omitting the temperature conditioning of the preform, as disclosed in the Japanese Laid-open Patent Publication No. 4-214322.
In both injection stretch blow molding methods, a preform is molded by using an injection mold composed of a cavity die for molding the outside wall of the preform and a core die for molding the inside wall of the preform, and a lip mold for molding the mouth portion of the preform, and the cavity space between the cavity die and the core die is filled with molten resin inserted into the cavity die by penetrating through the opening of the cavity die and the closed lip mold by injecting the molten resin from the bottom of the cavity die.
To release the injection molded preform from the injection mold, both the core die and lip mold are moved upwardly, or the core die is moved upwardly and the cavity die is moved downwardly, and the preform, after being released, is transferred into the blow mold while the mouth portion of the preform is held by the lip mold.
The reason why the preform is released by being drawn out also from the core die is that it is extremely difficult to stretch the preform in the axial direction by a stretching rod in the state of tightly holding the core die by shrinkage of the preform due to cooling, and therefore the preform is drawn out from both the cavity die and the core die using the lip mold also as the transfer member, thereby transferring into the blow mold.
Usually, when releasing the injection molded product from the injection mold, withdrawing of the injection molded product from the core die is more difficult than withdrawing from the cavity die. This is because the surface of the outside wall of the molded product is parted from the surface of the cavity die due to shrinkage by cooling of the injection molded product at the cavity side, while the inside wall of the molded product tightly contacts the core die due to shrinkage to the core die side.
The strength of the hot preform in a flexible state that can be processed by stretch blow molding is only enough to maintain the shape of the preform by the surface skin layer formed on the inside and outside surfaces of the preform. This is different from the ordinary injection molded product which is completely rigid as a consequence of complete cooling and solidifying. Therefore when drawing out the core die by holding the cooled and solidified mouth portion of the preform by the lip mold, unless there is enough strength to disconnect the skin layer of the inside wall of the preform from tightly contact with the core die surface, the preform is drawn out from the cavity die in tight contact with the core die, and squeezed by the lip die in fixed state, deforming as if bellows were being contracted, and thereby losing the shape of the preform.
Accordingly, an injection stretch blow molding method is designed to release the preform after cooling the preform to a temperature so that the skin layer of the preform surface is rigid enough to withstand the withdrawing force of the core die while stretch blow molding is possible. Although the cooling temperature varies with the thickness and design of the required preform, when a preform of a container with a wide mouth in which blow-up ratio is not so large, the draft of the cavity die and core die can be set large. The contact force of the preform inside from shrinkage can be alleviated by the draft, release at high temperature is enabled, and shrinkage due to cooling decreases, and owing to synergistic action release, releasing is easier than in the case of a preform of a container with a narrow mouth.
However, in the case of a preform of a container with a narrow mouth such as a bottle whose aperture is small, size is long, and blow-up ratio is large, the draft of the cavity die and core die is limited, and the limit becomes stricter as the length is greater. It is hence necessary to release the preform from the injection mold by cooling the preform to the compatible temperature enabling both release and subsequent stretch blow molding. This compatible temperature is, at ordinary temperature, 60 to 70 deg. C. in the case of polyethylene terephthalate, and 90 to 100 deg. C. in the case of polypropylene. In these temperature ranges, both release and stretch blow molding can be carried out.
However, in the case of a preform of polyethylene, which is excellent in thermal conductivity and high in molding shrinkage as compared with polyethylene terephthalate and polypropylene, when cooled to drawable temperature, the skin layer is formed thicker than necessary, and the internal high temperature region becomes narrow. Therefore if the preform is immediately transferred into the blow mold and processed by stretch blow molding, it is not swollen sufficiently by air pressure. At a temperature at which stretch blow molding is estimated to be possible, the preform remains tightly in contact with the core die, and when the core die is drawn out in this state, the preform is extremely deformed.
Therefore, with polyethylene preform, setting of the compatible temperature enabling both release and subsequent stretch blow molding is more difficult than in the case of polyethylene terephthalate or the like, and stretch blow molding by using the hitherto employed releasing means was extremely difficult even in the case of a container with a wide mouth.
It may be also considered to perform stretch blow molding by reheating and conditioning the preform after releasing to temperature suited to molding, but this requires experience, time, and energy, and temperature unevenness is likely to occur. Therefore, if temperature conditioning is employed, the injection stretch blow molding of polyethylene involves many technical difficulties.
Concerning polyethylene, aside from releasing, the difficulty also lies in the temperature for stretch blow molding the preform. In a blow molding method, the resin temperature for blow molding polyethylene is 175 to 200 deg. C. In an injection stretch blow molding method, such resin temperature is the molding temperature of the preform, and the temperature of a preform cooled so as to be self-supporting is extremely lower than such resin temperature.
When stretch blow molding the preform released at high temperature into a container such as a bottle before the surface temperature of the preform from the internal heat reaches the peak temperature, the time the surface temperature of polyethylene reaches the peak temperature is earlier than in the case of polyethylene terephthalate, and stretch blow molding polyethylene is difficult as compared with polyethylene terephthalate. It is hard to obtain a molded product excellent in wall thickness distribution near the peak temperature.
As a difficulty of stretch blow molding, it is estimated, since the thermal conductivity of polyethylene is higher than that of polyethylene terephthalate and polypropylene, transferring the internal heat of the preform to the surface of the preform after being released is relatively quick, the volume occupied by the internal high temperature portions contributing to the stretch blow molding becomes narrow earlier, and hence the internal heat quantity necessary for stretch blow molding tends to be insufficient. Hence, even in the case of a container with a wide mouth which is usually easier to release than a container with a narrow mouth, a favorable molded product can not be obtained unless stretch blow molding is done within a limited time (within temperature range).
Reaching peak temperature differs somewhat depending on the wall thickness of preform, design or molding conditions, and better products are not obtained unless stretch blow molding is adjusting accordingly. Therefore in the case of polyethylene, a severer molding operation is required than in the case of polyethylene terephthalate.
The present invention is devised in order to solve the problems in a method for injection stretch blow molding of polyethylene, and it is an object to present a novel method enabling to released a preform at a surface temperature compatible for both releasing of preform and subsequent stretch blow molding, by disconnecting the core die and the surface of the preform in advance by the use of pressure of a gas, and to perform stretch blow molding in a specified temperature range preferable for polyethylene.